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Serena Williams posts sweet tribute to her late dog, Jackie

On Instagram, the tennis champ posted multiple photos of her dog, Jackie Baila Pete Williams, who died this week at age 16.

In a series of Instagram photos posted Tuesday, tennis superstar Serena Williams mourned the loss of her adorable dog, Jackie Baila Pete Williams, who Williams said died this week at age 16.

"Today really is hard for me," reads the caption on a collage of Williams with Jackie, the first of three posts. "My special friend in which I got at 17 (2 weeks before I won my very first Grand Slam) left me today. She was 16 years young and up until a few days ago was still sprinting. Her poor body gave out this morning and she had a way of telling me it was time for me to be brave and let her go. With my dad by my side we were able to say a loving goodbye."

Williams, 34, added that she had been taking care of her "#bff" since 1999.

"I miss her so much," she told her 2.4 million Instagram followers. "I got out of the shower this afternoon and she was not there to lick my leg as she always did everyday to remind me how much she loved me. I feel so lucky to have such a special friend. Give your dog, cat, pet a big hug."

The 21-time Grand Slam singles champ's tribute continued with two additional Instagram posts.

"My angel," reads a caption accompanying a tennis-court photo of Jackie, adorably making herself comfortable in Williams' unzipped equipment bag.

A subsequent collage of Jackie shows the canine companion in repose, lounging by herself and with Williams' other dogs, Lauralei and Chip. That caption addresses Jackie directly, telling the late dog that "Lauralei, Chip, and Mommy miss you so much."

In that same caption, Williams offered a heart emoji while assuring Jackie that she would not be forgotten, adding, "thank you for the kindness and joy you brought us."

It's been an up-and-down 2015 for Williams, who arguably had the best year of her tennis career. Winning 26 of 27 matches in Grand Slam singles competitions, she won the Australian Open, French Open and Wimbledon before advancing to the semifinals of the U.S. Open.

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She also made news off the court. In July, "Harry Potter" author J.K. Rowling hailed her for her prowess as an athlete, role model and woman, then ripped into a Twitter user for body-shaming the tennis champ. In August, Williams chatted about her incredible career with TODAY's Tamron Hall — and teased the co-anchor about her backhand. And earlier this month, Williams told fans of her Facebook page that she'd chased down a man who tried to steal her phone.

Follow writer Chris Serico on Twitter.